0S , 
1e 
31 
Types of photogrammetric instruments used 
  
Whether vertical or convergent photography is used, optical train instru- 
ments or double projection instruments are employed as available and/or desired. 
The optical train instruments are used within limits, for bridging sup- 
plemental horizontal and vertical control. 
The double projection instruments are used within limits for bridging hor- 
izontal control by radial plot using templets which are made of strong flat- 
lying material in which can be cut circular holes centered on nadir points and 
slots centered on all radial points. The nadir and radial points utilized are 
finite image-point projections in the stereoscopic models which are scaled and 
leveled as accurately as possible. Scale for this initial setting up of the 
stereoscopic models to prepare the radial templets at mapping scale is estab- 
lished by use of a radial plot made in a previous sequence from contact scale 
prints of the photographs on scale-stable materials. These contact-print size 
templets are assembled at an appropriate, though much smaller scale than 
required for the mapping. Scale for this assembly is established by plane 
coordinate plotting of the basic horizontal control. Once the radial stereo- 
templets are prepared, they are assembled over an accurate plane coordinate 
grid. On this grid, the basic horizontal control points of the survey project 
were plotted at the scale to which the maps are to be compiled. This basic 
control consists of identifiable image points on each stereoscopic model which 
were accurately position-surveyed on the ground and are not farther apart than 
4 or 5 stereoscopic models. 
The double projection instruments are also used, within limits and as 
desired, for bridging supplemental vertical control. When this is done, photo- 
graphs, preferably about one-half the scale of the photographs which will be 
used for map compilation, are employed, and occasionally photographs as small 
as 1/3 that scale. This procedure, in actuality, may not properly be called 
bridging because the smaller scale photographs, leveled and scaled to accurate 
ground survey control (horizontal and vertical), are merely used to measure 
photogrammetrically the elevation of a sufficient number of identifiable image 
points for leveling the larger scale photographs when the mapping is being 
accomplished. 
As desired, optical train and double projection instruments are used for 
topographic mapping and making spot elevation measurements, and for plani- 
metric mapping and measuring profile and cross sections of base lines, designed 
centerlines, and field-staked lines. For use in conjunction with electronic 
computers, the profile and cross sections are measured for design and for 
measurement of pay quantities after construction is completed. The scale at 
which the maps are compiled with optical train instruments, depending on cir- 
cumstances and requirements, is anywhere from equality with the photography 
scale to a much larger scale. The practicable working scale limit at which 
maps are compiled is eight times the photography scale. 
The scale at which maps are compiled with double projection instruments 
is 4, 5, or 7 times the photography scale, depending upon the optimum projec- 
tion ratio of the instrument utilized. The largest scale at which maps can 
be compiled with these instruments is set by the relief height to flight height 
ratio in areas of rugged topography and in metropolitan areas where the build- 
ings are extremely tall. The relief height to flight height ratio (h/H) of 
some of the most commonly used double projection instruments is given in 
table 5. The h/H ratio is actually the vertical measurement range of the 
instrument divided by maximum projection distance. 
If the double projection instrument being used is one which has a pro- 
jection ratio of 5:1 and an h/H of 0.25, and the maximum relief height within 
one stereoscopic model is 440 feet (134 meters), the aerial photographs for 
mapping could not be taken from a flight height of less than 1,760 feet (536 
meters) above the low elevation in the stereoscopic coverage area. The optimum 
flight height from which stereomodel scale is determined, however, is less than